Numerical assessment of friction damping at turbine blade root joints by simultaneous calculation of the static and dynamic contact loads

Abstract

In turbomachinery, the perfect detuning of turbine blades in order to avoid high cycle fatigue damage due to resonant vibration is often unfeasible due to the high modal density of bladed disks.To obtain reliable predictions of resonant stress levels of turbine blades, accurate modeling of friction damping is mandatory.Blade root is one of the most common sources of friction damping in turbine blades; energy is dissipated by friction due to microslip between the blade and the disk contact surfaces held in contact by the centrifugal force acting on the blade.In this paper, a method is presented to compute the friction forces occurring at blade root joints and to evaluate their effect on the blade dynamics. The method is based on a refined version of the state-ofthe-art contact model, currently used for the nonlinear dynamic analysis of turbine blades. The refined contact model is implemented in a numerical solver based on the harmonic balance method able to compute the steady-state dynamic response of turbine bladesThe proposed method allows solving the static and the dynamic balance equations of the blade and of the disk, without any preliminary static analysis to compute the static loads acting at the contact interfaces.

Experimental Investigation on the Damping Effectiveness of Blade Root Joints

“…By substituting the real values of the test rig it is possible to obtain a value of the equivalent modal damping ratio generated by the eddy currents and associated to the first bending mode close to 5·10 −7 which is far below the material damping that is identified from the hammer test (10 −3 order of N 150 L 60 mm r 1 11.3 mm r 2 20 mm g 2 mm s 10 mm h 26 mm  6·10 7 1/m and active part of the blade (red area) for the determination of the damping coefficient due to eddy currents magnitude, half power method identification) and therefore it can be considered negligible.…”

“…In order to overcome the limitation of the above described linear modeling technique, non-linear methods have been developed in the last decade [1] to model the micro-slip behavior of blade/disk interfaces and their effect on the forced response of bladed disks. For these methods to be considered reliable and effective, two types of extensive experimental campaigns are necessary.…”

Hysteresis modeling of clamp band joint with macro-slip

“…The hysteresis behavior of jointed structures has been investigated extensively [1][2][3][4][5][6][7][8][9]. Those studies focused mainly on the bolted joints, where various hysteresis models, namely the Iwan model, Valanis model, Bouc-Wen model and Lu-Gre model, were adopted to simulating the joint hysteresis loops consisting of symmetric stick and slip regions under constant normal loads.…”

Nonlinear Forced Response of a Stator Vane With Multiple Friction Contacts Using a Coupled Static/Dynamic Approach

“…the amplitudes of oscillation are reduced increasing the fatigue life of the blade arrays. The major sources of friction damping in the bladed disk turbine are attributable to the blade-disc interfaces (blade root joint) [3,4], the contact between adjacent blades connected by interference at the tip (shrouds) or mid-span airfoil (snubber, [5]) and the presence of underplatform dampers [6,7]. Typical joints for vane segments are the so-called interlocking joints [8] located at the inner radius of the sector connecting adjacent sectors.…”